How Work Distribution Between the Stages of an Axial Multistage Compressor Affect Compressor Characteristics

One of the important factors affecting compressor performance is the distribution of work between compressor stages.

There can be many different axial compressors with the same design point value for efficiency, airflow and pressure ratio and all of these compressors will meet the required parameters at the design point. Compressors operate in a variety of modes. Many compressors are equipped with a turning inlet guide vane (IGV) or guide vane (GV). At the same time, the question of the number of stages with a turning IGV or GV, at what angle to turn them, and whether the maximum efficiency will be the same remains unresolved. To answer this, designers need to compare the impact of the distribution of work between compressor stages at the design point on the compressor characteristic. This allows you to choose the form of work distribution to achieve the specified parameters of the compressor in off-design modes of operation. This also allows you to obtain the required degree of pressure ratio and airflow at a given frequency of rotation of the compressor rotor. This analysis is necessary for jet engines since the compressor works in conjunction with the combustion chamber, turbine, and nozzle. This can also be carried out for other applications of multistage compressors.

With the help of AxSTREAM and ION, studies were carried out on multistage compressors with various shapes of the flow path, a different number of stages, and a different distribution of work among the stages.

Figure 1. Calculated typical characteristics of a multi-stage, variable compressor.

The problem is considered in the following formulation:

Influencing factor:

  • Distribution of work on compressor stages.

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Variable parameter:

  • Compressor operating mode.

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Axial compressor type:

  • With fixed IGV or GV.
  • With turn of IGV or GV.

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Additional task for the compressor with turn IGV or GV:

  • Develop a criterion for the required number of compressor stages with restagger angle of the blades IGV or GV.

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The compressor is taken from the article sourced at the end of this post

Figure 2. Diagram of map calculation in AxSTREAM ION
Figure 2. Diagram of map calculation in AxSTREAM ION

Calculation scheme in  AxSTREAM ION

The ION system was used to obtain the characteristics.

For example, the results of the calculation of a 6-stage compressor similar to the one described in the article are presented below.

Table 1
Table 1
Dependence of the degree of pressure ratio and the efficiency on the reduced rate for
Figure 3. Dependence of the degree of pressure ratio and the efficiency on the reduced rate for different laws of distribution of the load factor for the rss n = 1.0 for 6-stage compressor.

In Figure 3, different colors represent the calculation results for various forms of work distribution between the compressor stages.

It can be seen from the calculation that the efficiency of the compressor, even at the calculated point, depends on the form of work distribution between the compressor stages. It can also be seen that there is a difference in the pressure parameters and the degree of pressure ratio when the compressor is throttled at a constant rotor speed. This dependence is explained by the different form of distribution of work between the stages.

Dependence of the degree of pressure ratio and the efficiency on the reduced rate for
Figure 4. Dependence of the degree of pressure ratio in the compressor and the efficiency on the reduced airflow rate for different laws of distribution of the load factor for the relative shaft speed n = 0.80

Different colors in Figure 4 show the results of calculations for various forms of work distribution between compressor stages. It can be seen from the figure that when the compressor is throttled from n=1 to n=0.80, the characteristics are stratified in a fairly large range.  Each line in the figure corresponds to a certain form of distribution of work in the design mode and the compressor designed for it.

Since the requirements for the compressor at lower frequencies are set by the output system, it is possible to choose the distribution of work between the compressor stages in the design mode so that it is not necessary to turn the compressor blades from the design point to the 2nd point at low or high frequencies. This will allow you to choose the distribution of work between the stages, which will expand the range of operation of the compressor. Calculations must be made for a specific compressor. In this case, the nuances of the compressor flow path and the presence of supersonic stages were taken into account. Analysis and change in the distribution of work between compressor stages can reduce the number of transonic or supersonic stages, which can increase efficiency.

As seen throughout this blog, choosing the form of work distribution between the stages of an axial multistage compressor:

  • Expands the range of the compressor.
  • Determines the distribution of work, which achieves the maximum value of the compressor efficiency in off-design modes with or without turn of IGV or GV.
  • Reduces the number of supersonic or transonic compressor stages.
  • Reduces the number of adjustable compressor stages.
  • Obtains the required values of pressure ratio and MFR at a given lower or higher speed of the compressor rotor without the use of turn of IGV or GV.

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To learn more about how AxSTREAM can help you on your next compressor project, contact us at sales@softinway.com

Sources:

1) Ф.Ш.Гельмедов, Е.И.Степанов. Совершенствование параметров высоконапорных широкохордных ступеней осевых компрессоров и эффективность их использования при разработке компрессоров ГТД./ Научный вклад в создание авиационных двигателей. В двух книгах. Книга 2 Н34/ Колл. авторов; под общей научной редакцией В.А. Скибина и В.И. Солонина. –М.: Машиностроение, 2000, С115-138

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